Waves

Normal is called

Axis of Equilibrium

Axis of Equilibrium

Undisturbed position of the medium

Mechanical waves vs EM waves

Mechanical waves need a physical medium to pass through, whereas electromagnetic waves do not

Period

Time taken for a point on a wave to complete a full cycle

Inversely proportional to frequency

Polarisation

when oscillation is restricted to a single plane

Polaroid filters block?

Light that oscillates parallel to the polymer chains

When does partial polarisation take place?

In reflection

Polarised sunglasses?

Horizontal polymer chains

Reduces the glare from road water etc…

LCD layers

MIrror/Backlight

Vertical polarisation filter

Pass through liquid crystal

Liquid crystal causes 90 degree flip in polarisation

Passes through horizontal filter

Out the front glass

How does liquid crystal work?

If electrified, electricity causes crystal structure to change, no longer flips the polarisation

If not electrified, flip in polarisation takes place

Reflection of a wave leads to

Flip in polarity

The same as a 180 degree phase change

2 types of wave graph

Displacement time

Displacement distance

Superposition

When two or more waves meet the resultant displacement is equal to the sum of the individual displacements at that point

Destructive interference

Occurs when two waves that are in antiphase meet

They cancel eachother out entirely if they are at ½ wavelength or 180 degrees out of phase

The opposite of eachother

Constructive interference

Two waves that are in phase meet

They sum to make a larger value

The wave is reinforced or amplified

Coherent

Same amplitude, frequency and wavelength

Constant phase difference

Leads to a constant or repeated interference / superposition pattern

Are stationary waves coherent?

Yes

They do not travel in the same direction, but the relationship between their phase differences is constant - always changing from 0 to 360 out

Better way to describe waves in a SW

They have identical frequency, wavelength and amplitude

They travel in opposite directions with a constantly changing phase difference

Standing wave occurs when

The progressive wave and reflected wave superpose

Where does energy go to in a stationary wave?

Lost to the environment through dampening

It does not transfer any energy at either end

What happens when a wave is reflected?

The polarity changes or

It undergoes a 180 degree phase change or

The direction of displacement changes

1 cycle of a stationary wave

When 1 wave fully passes through another

Full oscillation of a point over what time period?

Period for half of a wavelength to pass

Leads to full flip of point from max positive to max negative

Not a whole wavelength because then it is at the same point of each wave again

When is stationary wave destructive

When multiple of a ¼ out of cycle - excluding halfs

As both waves are ¼ further

Totals for the resultant to be half out

So 180 degrees out of phase is entirely destructive

When is stationary wave destructive

Multiples of 1/2th

The two halfs combine to make a whole

Perfect overlap of each other leads to constructive interference

Relationship between frequency for first, second, third … harmonic

Second is 2x the frequency of first

Third is 3x frequency of first

Node

Formed when the relationship at that point is constantly destructive

Formed at the point where the wave is 1/2 out of cycle

So there is a half wavelength distance between nodes

So it is always destructive

what is root T/u equal to?

Wave Speed

f = 1/ lambda x C

1/2l = 1/lambda

f = 1/2L x root T/u

root T/u = wave speed = c

Harmonics in tubes

There must be a node at the closed end

There must be an antinode at the open end

Two closed ends = two nodes

Two open ends means it ends in an antinode on both ends

To calculate wavelength from harmonics in tubes

Find out how many distances there are between nodes

Distance between two nodes is ½ wavelength

e.g. 3.5 nodes = 3.5 × ½ wavelength

Refractive index

Measure of the optical density of a material in comparison ot vacuum

Relative refractive index is between two materials

Go into more optically dense

Refract towards the normal (potentially through)

Vice versa - less dense = refract away

Critical angle

The angle whern refracting into less dense, where it is refracted along the boundary

Incident < Critical

Refraction out of material

Incident > Critical

Total Internal Reflection